N-Zdistributions of secondary fragments and the evaporation attractor line

R. J. Charity
1998 Physical Review C  
The process of light particle evaporation moves the position of an excited fragment in the chart of nuclides towards a line which will be called the evaporation attractor line. The predicted location of this line is parametrized and the conditions necessary for the secondary fragment distributions to reach this line are discussed. ͓S0556-2813͑98͒03208-7͔ PACS number͑s͒: 24.60.Dr In many nuclear reactions, one or more excited primary fragments are formed which decay by the evaporation of
more » ... poration of nucleons and light clusters. This evaporation process can substantially alter the proton-neutron asymmetry of the initial primary fragments. In a study of complex fragment emission in fusion reactions, the Z values of the detected secondary fragments were measured ͓1͔. In order to estimate the average mass associated with each fragment Z, statistical model calculations were performed varying the assumed Z, A, and excitation energy of the primary fragments. It was noted that at sufficiently large excitation energies, independent of the assumed Z and A of the primary fragments, evaporation models predict that the average location of the secondary fragments in the chart of nuclides is always close to a particular line. The location of this line is mainly determined by competition between proton and neutron evaporation. For compound nuclei on the neutron-rich side of the line, neutron emission is the most important evaporation mode and this drives the system towards the line. On the neutronpoor side, proton emission is the strongest decay mode and again, this acts to move the decaying system towards the line. This line thus acts as if it is attracting the decaying systems and hence it will be called the evaporation attractor line ͑EAL͒. The same concept was referred to as the "residue corridor" by Dufour et al. ͓2,3͔ who indicated that its position is near the line where the ratio of neutron and proton decay widths (⌫ n /⌫ p ) is unity. For light systems, the attractor line is coincident with the line of ␤ stability. However, for heavier systems, the larger Coulomb barrier for proton emission pushes this line to the neutron-deficient side of the valley of stability. At the attractor, the neutron and proton driving forces are about equal. A system located on the attractor will tend to follow the attractor down to lower masses until its excitation energy is exhausted. At lower excitation energies, the only other important light particle decay mode is ␣ particle evaporation. The emission of this particle moves the decaying system almost parallel to the attractor line and thus preserves a memory of system's neutron excess or deficiency. For these low excitation energies, ␣ particle emission is often less probable than neutron and proton evaporation and only slows down the general movement towards the attractor. The calculations and conclusions of Ref. ͓1͔ were confined to region of lighter fragments (ZϽ40). The location of the EAL in this region of the chart of nuclides was subsequently found to be coincident with the EPAX formula of Sümmerer et al. ͓4͔ giving the average location of fragmentation products produced in reactions of GeV protons on target nuclei located close to the line of ␤ stability. These products can therefore be understood as the evaporation residues associated with the decay of highly excited primary fragments produced by the initial reaction with the proton ͓4͔. In light of these conclusions and the need to estimate the mass of fragments with larger Z values, evaporation calculations were performed to cover all of the known region of the chart of nuclides. The conclusions from these calculations are identical to those obtained early apart from two exceptions. First for primary fragments that are very neutron-rich relative to the EAL, the location of the secondary fragments approaches, but never reaches, the attractor line even for very large excitation energies. Also it was determined that for heavier nuclei, the EAL and the EPAX formula for fragmentation products are no longer identical. These two results are related and easily understood ͑see later͒. Statistical model calculations were performed with the computer code GEMINI ͓5,6͔. Apart from the dominant lowenergy decay modes ͑n, p, and ␣ emission͒, the calculations also consider the evaporation of ground and excited states (E*Ͻ5 MeV) of all He, Li, and Be isotopes. Separation energies were obtained from the experimental and predicted masses tabulated in Ref. ͓7͔. The other parameters used in these calculations are identical to those used in Ref. ͓8͔. The location of the evaporation attractor line is determined from evaporation characteristics of the system near the end of its decay path, it is not sensitive to uncertainties in the values of the statistical model parameters at the highest excitation energies. However, these parameters are important when predicting the mass distributions of the fragments. The EAL is sensitive to neutron and proton separation energies, however, except for very heavy systems, the attractor line lies in a region where atomic mass excesses are well known experimentally. The EAL is also sensitive to the Coulomb barriers for proton emission. A number of studies have suggested that average Coulomb barriers for charged particle emission are lower than those obtained from optical model fits ͑Ref. ͓8͔, and references therein͒. However, at present it is not clear whether this is only a high excitation energy phenomenon or to the extent to which the evaporation of,
doi:10.1103/physrevc.58.1073 fatcat:vcbd25bx6rh7dac7ji4elwpdry